At present, there are two basic fabrication routes for the production of Nb3Sn superconducting wire. The most common is the “bronze route”, so called because it features Nb filaments processed in a bronze (copper-tin) matrix. Bronze route wire is responsible for the bulk of Nb3Sn wire production in the world today. It is popular because despite the need for intermediate anneals, the production process is rather straightforward and amenable to large lot sizes. For uses requiring higher superconducting critical current levels, the “internal tin” process, so called because the tin is separate from the copper until the final heat treatment step, is used because it can deliver several times the supercurrent at high magnetic fields compared to the bronze process wires. This is because the internal tin process allows the creation of wire having more tin, and thus the capability to provide more Nb3Sn in the final wires' cross section. This invention pertains to improvements in the “internal tin” method of Nb3Sn wire production.
An important performance measure for superconducting wire is the critical current density, Jc, which is defined as the maximum electric current a wire can carry divided by the cross sectional area (or some defined fraction of the area) of the wire. A common form for expressing the critical current density is the non-copper critical current density, where the dividing area is all but the stabilizing copper. The Jc of Nb3Sn superconducting strand made by the “internal Sn” process (which is primarily a composite made of Cu, Nb, and Sn and/or their alloys) strongly depends on the fraction of Nb and Sn available in the wire cross section. Generally, the higher the fraction of Nb and Sn within the wire, the higher is the fraction of the wire that can be converted to the Nb3Sn superconducting phase by strand heat treatment. As a result, modern designs for high Jc Nb3Sn strand made by the internal Sn process consist of high Nb and Sn fractions, and a low amount of Cu.
Although a wire with the highest theoretical Jc would therefore be made of only Nb and Sn in a stoichiometric 3:1 atomic ratio (since this would maximize the amount of Nb3Sn in the cross section and minimize the fraction of non-superconducting Cu), in practice a certain amount of Cu is required in the cross section. The copper within the superconducting package or “subelement” serves several purposes, including:                1. Cu makes the wire easier to process because it has a hardness level between that of harder Nb and softer Sn. Cu is thus placed amongst the filaments, between the Sn core and Nb filaments, and between the subelements, to aid in the drawing process.        2. A small amount of Cu is needed to reduce the reaction temperature required for converting the Nb and Sn to Nb3Sn. This is desirable for obtaining Nb3Sn microstructures that result in high Jc, and is also desirable from a device manufacturing point of view.The Cu also has an additional function, one that is relevant to the present invention:        3. Cu between the Nb filaments serves as a path for diffusion of Sn, to allow the Sn source to be dispersed throughout the subelements and to all of the Nb filaments. Having adequate Sn locally available to all Nb filaments in a wire during heat treatment is important for reacting the Nb to Nb3Sn, and obtaining a Nb3Sn microstructure that results in high Jc.        
Thus the problem of designing high current density Nb3Sn wires is reduced to incorporating the optimum ratio of Nb, Sn, and Cu components in a package that can be fabricated and heat treated to produce practically useable strand that will be electrically stable as supercurrent approaches its critical value (i.e., so that small inhomogeneities will not cascade the loss of supercurrent appreciable short of its upper bound value, known as a “quench”). The present invention prescribes a design of such a wire and method for producing same. Although many individual components of this invention may have been part of prior art or known in the industry, it is the unique summation and synergistic integration of all the concepts that produces the high critical current density. Some past designs such as the “tube process”, as in Murase U.S. Pat. No. 4,776,899, have very high values of Sn wt %/(Sn wt %+Cu wt %) within the diffusion barrier, and other designs had fine filaments with low LAR (infra); and other designs have had distributed diffusion barriers, which is defined as diffusion barriers around each individual subelement separated by copper instead of a single diffusion barrier encasing all subelements; but none addressed all the issues that are critical for effectiveness and provided a solution to such issues. The proof of this uniqueness is that despite many of these individual concepts dating back to mid 1970's, starting with Hashimoto U.S. Pat. No. 3,905,839, the present invention representatively results in a non-copper critical current density of about 3000 A/mm2 at 4.2K, 12 Tesla and about 1700 A/mm2 at 4.2K, 15 Tesla, which is an improvement of about tenfold from the initial invention of internal tin superconductor wire and approximately 50% increase from the prior art values of the late 1990's.